In the field of motors with propulsion members which are moved by means of the supply of a fluid and are therefore generally defined as hydraulic, it is known of the possibility of provide the propulsion members with a cylinder and a piston which are telescopically coupled together so as to translate relatively upon rotation of an eccentric cam associated with the drive shaft, thus providing thrust to the shaft itself.
Said propulsors may be arranged radially or inclined as described Ser. No. 09/01708 filed Feb. 2, 1998 in co-pending patent application in the name of the same Applicant.
It is also known that one of the problems posed by these propulsors is the need to keep the end edge of the cylinder and the piston sealingly adherent with respect to the said eccentric cam and a reaction element consisting of a cover-piece fixed to the casing of the motor in the case of radial motors or a disc keyed onto the drive shaft in the case of inclined propulsors, so as not to cause fluid leakages during the relative travel between piston and cylinder. One of the solutions commonly used to obtain this seal consists in the insertion, inside each propulsion member, of a resilient element, such as a helical spring for example, arranged coaxially with respect to the propulsor and designed to push against corresponding internal shoulders of the cylinder and the piston so as to press said cylinder and piston against the associated abutment surfaces.
An example of this type of embodiment is known from the U.S. Pat. No. 3,577,830 in the name of the same Riva Calzoni S.p.A. This solution, however, has some drawbacks including those caused by the dynamic stresses to which the spring is subjected during travel of the piston with respect to the cylinder, which results in the need for over-dimensioning of the spring itself, causing a strong thrust on the sliding surfaces making contact, with consequent greater wear of the latter.
In addition to this, the presence of the spring and the associated support shoulders for them inside the cylinder prevents a reduction in the volume of fluid which does not emerge from the cylinder at the end of the compression phase (so-called dead volume), increasing the problems of exchanging the fluid itself with fresh fluid supplied by the delivery ducts.